1. Field of the Invention
The present invention relates to a niobium-containing aqueous solution for use in producing a niobium-containing oxide catalyst. More particularly, the present invention is concerned with a niobium-containing aqueous solution for use in producing a niobium-containing oxide catalyst, wherein the oxide catalyst is for use in a catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase and is prepared by a process comprising mixing the niobium-containing aqueous solution with an aqueous mixture or aqueous mixtures containing compounds of active component elements of the oxide catalyst other than niobium, to thereby provide an aqueous compound mixture, and drying the aqueous compound mixture, followed by calcination. The niobium-containing aqueous solution of the present invention comprises water having dissolved therein a dicarboxylic acid, a niobium compound and optionally ammonia, wherein the dicarboxylic acid and the niobium are present in a specific molar ratio, and the optional ammonia, if any, is present in a small amount. By the use of the niobium-containing aqueous solution of the present invention, it has become possible to produce efficiently an oxide catalyst which, when used in a catalytic oxidation or ammoxidation of propane or isobutane in the gaseous phase, exhibits high catalytic activity. Therefore, by the use of the oxide catalyst produced by using the niobium-containing aqueous solution of the present invention, (meth)acrylic acid or (meth)acrylonitrile can be produced in high yield. The present invention is also concerned with the oxide catalyst produced by using the niobium-containing aqueous solution of the present invention, and the use of the oxide catalyst.
2. Prior Art
There has been well known a process for producing (meth)acrylic acid by oxidation of propylene or isobutylene and a process for producing (meth)acrylonitrile by ammoxidation of propylene or isobutylene. Recently, as substitutes for such processes for the oxidation and ammoxidation of propylene or isobutylene, attention has been attracted to a process for producing (meth)acrylic acid by catalytic oxidation of propane or isobutane in the gaseous phase (i.e., catalytic reaction of propane or isobutane with molecular oxygen in the gaseous phase) and a process for producing (meth)acrylonitrile by catalytic ammoxidation of propane or isobutane in the gaseous phase (i.e., catalytic reaction of propane or isobutane with ammonia and molecular oxygen in the gaseous phase). Further, a number of proposals have been made with respect to catalysts for use in the above-mentioned processes for the catalytic oxidation and catalytic ammoxidation of propane or isobutane in the gaseous phase.
For example, as a catalyst for use in the above mentioned processes for the catalytic oxidation and catalytic ammoxidation of propane or isobutane in the gaseous phase, oxide catalysts containing molybdenum, vanadium, niobium and tellurium are known. Such oxide catalysts are disclosed in U.S. Pat. Nos. 5,049,692, 5,231,214 and 5,422,328, European Patent Publication No. 529 853 B1 and Unexamined Japanese Patent Application Laid-Open Specification Nos. 6-227819, 6-279351, 7-10801, 7-144132, 7-232071, 7-289907, 7-315842, 8-57319 and 8-141401.
As further examples of oxide catalysts for use in the above-mentioned processes for the catalytic oxidation and catalytic ammoxidation of propane or isobutane in the gaseous phase, an oxide catalyst containing tungsten, vanadium, tellurium and niobium, is disclosed in Unexamined Japanese Patent Application Laid-Open Specification No. 6-228073; oxide catalysts containing molybdenum, vanadium, antimony and niobium, are disclosed in European Patent Publication No. 767 164 A1 and Unexamined Japanese Patent Application Laid-Open Specification Nos. 5-213848, 9-316023 and 10-45664; and an oxide catalyst containing molybdenum, vanadium, bismuth and niobium, is disclosed in U.S. Pat. No. 4,760,159.
With respect to sources of niobium, the above-mentioned patent documents describe the use of Nb.sub.2 O.sub.5, a niobic acid (Nb.sub.2 O.sub.5.nH.sub.2 O), Nb.sub.2 (C.sub.2 O.sub.4).sub.5, niobium oxalate, niobium tartrate, ammonium niobium oxalate, ammonium niobium tartrate, NbCl.sub.3, NbCl.sub.5, Nb(OEt).sub.5, Nb(O-n-Bu).sub.5 and the like. However, heretofore, no study has ever been made with respect to the relationship between the composition of the niobium source and the catalytic activity of the niobium-containing oxide catalyst obtained.
For example, the above-mentioned U.S. Pat. No. 5,049,692 discloses the use of a niobium compound (called "ammonium niobium oxalate") as the niobium source; however, currently, the composition of the "ammonium niobium oxalate" is not known, and U.S. Pat. No. 5,049,692 has no description about the composition of the "ammonium niobium oxalate", such as a molar ratio of oxalic acid to niobium.
The above-mentioned U.S. Pat. No. 4,760,159 discloses the use of an aqueous slurry of Nb.sub.2 O.sub.5 ; the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 9-316023 discloses the use of a solution of a niobic acid in a mixture of water and oxalic acid, wherein a large amount of oxalic acid is used (molar ratio of the oxalic acid to the niobium in the niobic acid: about 6.0 to 8.5); and the above-.mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 6-227819 discloses the use of a solution obtained by dissolving Nb(O-n-Bu).sub.5 in 1,4-butanediol. However, none of these patent documents describes the composition of the niobium source and the relationship between the composition of the niobium source and the catalytic activity of the obtained oxide catalyst.
The above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 7-315842 teaches that niobium compounds, such as niobium oxalate, niobium tartrate, ammonium niobium oxalate and ammonium niobium tartrate, have high solubilities in water as well as in aqueous solvents containing alcohols, organic acids or inorganic acids, so that these niobium compounds can be handled with ease; however, this patent document also has no description about the composition (such as a molar ratio of oxalic acid to niobium) of each of the above-mentioned niobium compounds and the relationship between the composition of the niobium compound and the catalytic activity of the obtained oxide catalyst.
The conventional oxide catalysts produced using the above-mentioned conventional niobium sources do not have satisfactory catalytic activities. Therefore, by the use of such conventional oxide catalysts, it is impossible to produce (meth)acrylic acid or (meth)acrylonitrile in high yield. Particularly, when the above-mentioned oxidation or ammoxidation is per-formed in a fluidized bed reactor using a carrier-supported catalyst comprising any of the above-mentioned conventional oxide catalysts, the carrier is likely to adversely affect the reaction, thereby leading to a lowering of the yield of (meth)acrylic acid or (meth)acrylonitrile.
Further, the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. 7-315842 teaches that a compound oxide catalyst (containing niobium) having high catalytic activity can be obtained by a method comprising: (1) mixing a niobium-containing aqueous solution with an aqueous solution containing compounds of active component elements of the oxide catalyst other than niobium to thereby obtain an aqueous compound solution; (2) removing the aqueous solvent from the aqueous compound solution by, for example, spray drying or freeze drying before the precipitation of the insoluble matters occurs, to thereby obtain a solid catalyst precursor; and (3) calcining the obtained solid catalyst precursor. However, there are only 10 minutes between the mixing in step (1) above and the occurrence of the precipitation. Therefore, the technique of this patent document is difficult to practice on a commercial scale.
Therefore, it has strongly been desired to develop an oxide catalyst which not only can be advantageously used for producing (meth)acrylic acid or (meth)acrylonitrile in high yield, but also can be efficiently produced on a commercial scale.